FIG. 1 is an example of a basic ground fault circuit interrupter (GFCI) for the detection of AC residual currents, also known as a residual current device (RCD). The operation of such circuits is well-known so only a brief description will be given.
A single phase AC mains supply to a load LD comprises live L and neutral N conductors which pass through a toroidal ferromagnetic core 10 of a current transformer CT1. The conductors L, N serve as primary windings of the current transformer CT1 and a winding W1 on the core serves as a secondary winding. In relation to the primary conductors, the term “winding” is used in accordance with conventional terminology, even though these conductors pass directly through the core rather than being wound on it. The neutral conductor N is grounded.
The currents IL and IN in the live and neutral conductors L, N flow in opposite directions through the core 10. Thus, under normal conditions, the vector sum of the primary currents IL and IN is zero in the absence of an earth fault (residual) current Ig caused by, for example, a person touching the live conductor L. However, the presence of an earth fault current Ig leads to a differential current IΔ in the CT1 primaries, i.e. a non-zero vector sum of the primary currents IL and IN, which induces a mains frequency current in the secondary winding W1.
The output of the secondary winding W1 is fed to a differential current detector circuit 20. The circuit 20 may be a WA050 RCD integrated circuit (IC) powered from the mains supply (the connections to the mains supply are not shown). The IC 20 is an industry standard RCD IC supplied by Western Automation Research & Development Ltd, Ireland. If IΔ exceeds the operating threshold of the IC 20, the IC 20 will produce an output signal on line 90 which will cause an actuator 30 to open ganged switch contacts SW in the live and neutral conductors L, N to disconnect the mains supply from the load LD. The actuator 30 typically comprises a solenoid-based electromechanical switch, such as a relay, and associated switching circuitry, the current flow through the solenoid either being increased to above a threshold, or reduced to below a threshold, to open the contacts SW, depending on the type of circuit. This is well known.
The GFCI also includes a test circuit connected at one end to the live conductor L upstream of CT1 (upstream means the direction away from the load LD) and at the other end to the neutral conductor N downstream of CT1. The test circuit comprises a resistor Rt connected in series with a normally open test switch T. By momentarily pressing T to close the contacts a part of the live current is diverted to neutral and bypasses the core 10, thereby simulating an earth fault current for detection by the detector circuit 20.
FIG. 2 demonstrates a problem associated with the basic GFCI of FIG. 1. In FIG. 2 the test circuit is present, but is not shown. Also, in FIG. 2 the components 20 and 30 are shown as a single item to avoid overcomplicating the figure.
FIG. 2 shows a ground to neutral fault condition 40 arising from an insulation breakdown between the load side neutral conductor N and ground. Because the neutral is already grounded at the origin of the supply, a second grounding of the neutral at the load is referred to as a DGN fault—double grounded neutral fault. This condition could arise due to miswiring or insulation breakdown. If a person touches a live conductor under a DGN condition a current Ig will flow through the person's body to ground. The fault current Ig will now split into two currents, Ige and Ign, with Ige flowing back to the supply via the earth or ground return path, and Ign returning back to the supply via the DGN fault connection 40 and the neutral conductor N. The ratio or division of these two currents will depend on the impedance of the neutral and ground paths. In many installations, the ground wire could be of smaller cross sectional area than the neutral wire with the result that Ige will be smaller than Ign. In any event, CT1 will no longer see the total fault current and will only see Ige, and if Ige is less than the trip threshold of the GFCI, the device will not trip.
UL943 requires GFCIs to trip automatically or continue to provide protection under a DGN condition. FIG. 3 shows one example of how this can be achieved, but other arrangements are also used in practice.
In the arrangement of FIG. 3, there is a second current transformer, CT2, having a winding W2, similar to the winding W1, on its core. An oscillator circuit 50 is connected to the winding W2 and produces a continuous AC current in the winding W2. Under normal conditions, this current has no effect. However when a DGN fault 40 occurs, a loop is formed by the neutral and earth paths, and a current Iosc is induced into this loop, as indicated by the arrows in FIG. 3 (in this case W2 acts as the primary winding and the neutral conductor N the secondary winding). CT1 sees Iosc as a differential current in the neutral conductor N which is representative of a ground fault current above the operating threshold of the GFCI. This differential current is detected by the differential current detector 20 which in turn produces an output to the actuator 30 which causes the GFCI to trip contacts SW to open). The DGN detecting circuit can function without an earth fault and without a load connected. CT2 can be referred to as a DGN CT. The oscillator frequency can be set at any value suitable for detection by the detection circuit 20. In a GFCI it is normally set in the low KHz range (e.g. 1-2 Khz) to avoid nuisance tripping at the 6 mA level.
Since the function of CT2 is to induce an oscillatory current into the neutral conductor N, actually the core of CT2 need only surround that conductor. It is shown surrounding both load conductors L and N since the CT assembly is normally commercially supplied as a combined pair, so it is not practical to pass the neutral conductor N alone through one CT and not the other. However, a separate DGN CT could be used, in which case the neutral conductor N alone could be passed through it. This is also the case in FIGS. 4 to 6.
All GFCIs have a test circuit which enables the user to manually test the device. The test circuit, which again comprises a test resistor Rt connected in series with a normally open switch T, is connected at one end to the live conductor L upstream of CT1 and at the other end to the neutral conductor N downstream of CT2. The value of the resistor Rt is selected to generate a test current within the prescribed limits of the product standard, which for a GFCI based on UL943 is 9 mA at rated voltage. When the test switch T is closed, a differential current is caused to flow from live, through Rt and the test switch T to neutral. The test switch and the test resistor have to be rated at full mains supply voltage, and for a single test winding the test resistor may need a power rating of about 1 watt (i.e. 120V×9 m˜1 W). These ratings place demands on the components in terms of size, cost and reliability, etc. However, a major problem with conventional GFCI test circuits is that the correct operation of the DGN function is not verified, leaving the user at risk in the event of failure of the DGN detection circuit.
It is the purpose of this invention to avoid or mitigate some or all of the above problems.